Jet tube sheet dryers, commonly known as jet tube dryers, are used to dry sheets such as wood veneer.
A jet tube sheet dryer of the type manufactured by The Coe Manufacturing Co. of Painesville, Ohio, (hereinafter referred to as a "Coe jet tube dryer") typically comprises twelve to twenty-five dryer sections, each of which has an air-delivery end and a chain drive end. Each dryer section contains four dryer decks, and each dryer deck has six pairs of elongate, vertically-opposed, horizontal jet tubes. Between each two adjacent pairs of jet tubes is a pair of vertically-opposed, horizontal veneer rollers. The veneer rollers define the path of movement of sheets of veneer through the dryer section. The jet tubes and the veneer rollers extend longitudinally between the air-delivery end of the dryer section and the chain drive end thereof. Each jet tube is of generally rectangular cross-section and has vertical walls which taper longitudinally from a large open end to a small closed end. Each tube has a flanged collar on the open end and a mounting bracket on the closed end. Lateral frame members are provided at the air-delivery end at the levels of the different dryer decks, and at the chain drive end there are lateral frame members intermediate the dryer deck levels. The green (wet) veneer is supported and moved through the dryer by the roller pairs driven by a chain and sprocket arrangement. The jet tubes are installed in the dryer section framework with their open ends at the air-delivery end of the dryer section and their closed ends at the chain drive end. A longitudinally-extending bottom side of the upper tube of a pair faces a parallel, longitudinally-extending top side of the lower tube of the pair. These parallel, facing, longitudinally-extending sides each have a plurality of holes formed therein. Fans, located above the dryer section, force heated air down toward a dryer door, located a few feet from the air-delivery end of the dryer section. The door deflects the heated air into the open end of the jet tubes. Thus, pressurized heated air is directed at the green veneer moving through the dryer from the holes in the jet tubes. The partially-enclosed area between the dryer doors and the air-delivery end of the dryer functions as a plenum where pressure is built up because of the greater capacity of the fans to direct heated air to the plenum than of the jet tubes to bleed off heated air from the plenum. The holes serve to maintain a generally uniform pressure throughout the length of the tube in an airstream flowing therethrough.
In most jet tube sheet dryers, each jet tube has a flanged collar on its open end. The tubes are installed into the dryer from the air-delivery end thereof and the flanged collars overlap dryer frame members at the air-delivery end. Retainer plates are placed over the flanges and are attached to the frame members of the dryer. The retainer plates prevent withdrawal of the jet tubes from the dryer. In addition, the flanges and the retainer plates together serve to substantially block passage of forced air through the dryer otherwise than by way of the interiors of the jet tubes.
The Coe Manufacturing Company manufactures two main models of jet tube dryer, namely the M-62 and the M-72. In each case, the tube is 15'41/2" long and about 6" wide. However, the open end of the jet tube for the M-62 dryer is about 6 inches high, whereas the open end of the tube for the M-72 dryer is about 7.5 inches high. Therefore, jet tubes for the two dryers are not interchangeable. Moreover, seven different flange designs are employed in the M-62 dryer and three different flange designs are employed in the M-72 dryer. Accordingly, if a mill has both M-62 dryers and M-72 dryers, it is necessary to carry several different jet tubes in inventory if down time is to be minimized.
One of the major malfunctions that occurs in a jet tube sheet dryer is known as a plug-up. In a plug-up, the flow of veneer is blocked. A plug-up may be caused by distortion of a jet tube, e.g., sagging of the upper jet tube, so that it obstructs the veneer path, or by failure of the bearings of the veneer rollers, resulting in the track of the veneer being altered. When a plug-up occurs, a fire may ensue, causing greatly elevated temperatures in the dryer. If the fire is put out using water, the temperature change may generate substantial residual stresses in the jet tubes, causing warping. Obviously, the dryer is out of service until the fire is put out and any necessary repairs have been made.
The jet tubes manufactured by The Coe Manufacturing Company for use as original equipment in the M-62 and M-72 jet tube sheet dryers comprise a wide channel section component and two narrower channel section components. The two narrower components are welded together and are fitted in the open side of the wide component. This type of tube is complex and expensive to manufacture.
A jet tube may be manufactured to fit the Coe M-62 or M-72 jet tube sheet dryer from a generally trapezoidal sheet of steel that is folded along four longitudinal fold lines so as to bring the two edges of the sheet, that extend longitudinally of the tube, together, and the two edges are lock seamed and tack welded together. Because this type of tube is made from a single sheet of steel, residual stresses are set up on the material during manufacture. Therefore, these tubes cannot readily be straightened after warping even by breaking the weld, because the residual stresses cannot be fully released.
It frequently occurs that during operation of a jet tube sheet dryer, debris accumulates inside the jet tubes, reducing the efficiency of the dryer and increasing the danger of fires. In order to remove debris from a jet tube of a conventional M-62 or M-72 jet dryer, it is normal to introduce a high pressure jet of water into the tube by way of its large end, and thereby force the debris from the tube by way of the holes in the sides facing the veneer path. However, the water jet tends to compact the debris at the small end of the tube. In some dryers there is an opening at the small end of the jet tube to allow the water jet to eject the debris from the tube by way of the opening. This expedient has not proved successful, because in order to avoid reducing the airflow towards the veneer it is necessary to provide the opening with a removable plug, but the plugs are frequently mislaid and therefore cannot be replaced after cleaning is completed. Moreover, because the opening is quite small, there are spaces that are not disturbed by the flow of water through the tube and debris tends to accumulate in those spaces.
Attempts have been made to clean this type of tube by backwashing: inserting a pipe through the hole at the small end of the tube and delivering water into the tube by way of the pipe. This method also results in packing of debris in the small end of the tube. Consequently, it is frequently necessary to remove the tubes from the dryer in order to clean them. Each tube in turn is removed from the dryer, stood vertically with its large end down, and banged so as to displace debris from the interior of the tube. Since each tube is over 15 feet long and weights about 50 pounds, and a typical jet tube dryer includes more than 500 tubes, it will be seen that it is a time-consuming task to maintain the tubes free of debris.
All Moore International jet tube sheet dryers that were in commercial use prior to 1968 employed vertical frame rails to support the jet tubes. At each end, the jet tube was positioned between two adjacent vertical frame rails and was supported at its sides by those rails. However, due to the weight of the dryer the vertical frame rails tended to warp and buckle. If two adjacent rails buckled together, it became impossible to insert a jet tube between the rails until the rails had been straightened. In 1968, Moore International abandoned use of vertical frame rails, in favor of horizontal frame rails. A bolt which is secured to one of the horizontal frame rails engages a sleeve which projects from the small end of the tube in order to support the tube at the chain drive end of the dryer. The jet tubes of all jet tube sheet dryers manufactured after 1968 are supported at their small ends by engagement of a member projecting from the small end of the tube with a member that is stationary relative to the support frame.
During the period from 1962 to 1968, Moore International manufactured a jet tube sheet dryer in which the jet tube was provided with a removable end cap at its small end. The end caps were held in position by a pin which extended vertically through the small ends of two adjacent tubes and the caps received by those tubes. It was found to be difficult to insert the pins, since each pin had to pass through eight holes and the warping of the tubes caused misalignment of the holes and bending of the pins. Therefore the end caps frequently were not replaced, and consequently the dryer ceased to function correctly. It was not possible to use removable end caps on the jet tubes in a dryer with horizontal frame rails, because of the member projecting from the small end of the tube. Therefore, use of jet tubes with removable end caps was abandoned in 1968.
The flanges on the jet tubes that are conventionally provided for the Coe M-62 dryer are so wide that they partially cover the bearings at the air-delivery end of the dryer. Consequently, it is difficult to inspect the bearings except when the tubes are removed, and so the bearings are not inspected sufficiently often. Because the bearings cannot be inspected readily, they do not receive proper maintenance and are subject to relatively frequent failure.
It has been proposed in U. S. Pat. No. 4,428,128 issued Jan. 31, 1984 (Coulson et al) that jet tubes without flanges be employed. However, this solution is subject to the disadvantage that the absence of flanges allows air to leave the plenum without entering a jet tube, by passing between the tubes.